1. Field of the Invention
The present invention relates to a vehicle brake device in which normally opened control valves which are electrically controlled in such a manner as not only to be changed over between a fully opened state and a fully closed state but also to obtain a half opened state between the fully opened and closed states are interposed between a fluid pressure generating unit which generates a fluid pressure in accordance with an operation amount and wheel brakes which are mounted on road wheels.
2. Description of the Background Art
A vehicle brake device is known in, for example, Japanese Patent Unexamined Publication No. JP-A-2001-47988 in which normally opened control valves or proportional solenoid valves which are electrically controlled in such a manner as not only to be changed over between a fully opened state and a fully closed state but also to obtain a half opened state between the fully opened and closed states are interposed between a master cylinder which generates a fluid pressure in accordance with an operation amount of a brake pedal and wheel brakes in order to cause the braking operation of the wheel brakes to continue even after a brake pedal effort was been released when the vehicle stopped in association with application of brakes. In addition, a vehicle brake is known in, for example, Japanese Patent Unexamined Patent Publication No. JP-A-10-100876 in which the normally opened control valves are interposed between hydraulic lines which communicate with outlet sides of pumps which can operate independently from the application of brakes and connect to the wheel brakes and the master cylinder.
In the vehicle brake devices disclosed in the Japanese Patent Publications Nos. JP-A-2001-47988 and JP-A-10-100876, however, when attempting to hold a parking brake in an applied state for many hours, the normally opened control valves need to be held closed for many hours in order to hold brake pressure.
On the other hand, the normally opened control valve is required to have both a quicker response and a fine opening control, and in order to meet the requirements, the normally opened control valve is configured into a short-time rating valve by setting a small inductance for the coil. In such a short-time rating type normally opened control valve, the value of current flowing through the coil provided in the normally opened control valve becomes high, which is thermally disadvantageous, and the thermal capacity of a drive circuit such as a field-effect transistor (FET) which controls the opening of the normally opened control valve has to be set relatively large.
Due to this, in the brake system in which the normally opened control valves are kept closed for many hours in order to hold the parking brake in the applied state, the normally opened control valves become thermally less advantageous, and this calls for enlargement in size of the normally opened control valves and the drive circuit, resulting in an increase in costs.